Nuclear reactors are used for electric power generating facilities. Nuclear reactors include fast neutron reactors. Fast neutron reactors are nuclear reactors which mainly use fast neutrons to cause fission of fissionable nuclides to generate power. Sodium, lead-bismuth alloys, and other heavy metals and, further, gas are used to cool the core. In nuclear reactors of the prior art, fission occurs at the core as a whole to generate power.
The criticality of the core of a nuclear reactor is maintained and the output is adjusted by for example control rods. The control rods are formed by a material which easily absorbs neutrons. At the start of an operation cycle, when inserting control rods into the core, along with the progress in burning, the control rods are gradually withdrawn so as to maintain the critical state while maintaining output. In this way, in operation of a nuclear reactor, control is necessary for maintaining the criticality of the nuclear reactor. Control is performed for maintaining the criticality on a continuous basis from the start of the operation cycle to the end of the operation cycle.
Japanese Patent No. 3463100 discloses a nuclear reactor in which control for maintaining the criticality in the operation cycle is unnecessary. This nuclear reactor employs the burning method called CANDLE (Constant Axial Shape of Neutron Flux, Nuclide Densities and Power Shape During Life of Energy Production) burning. In CANDLE burning, the core can be divided into a generally new fuel part, burning part, and burned up part. The burning part moves toward the new fuel part by a speed proportional to output along with burning. In CANDLE burning, after a single operation cycle is ended, fuel is replaced for the next operation cycle. When replacing the fuel, the burned up fuel can be taken out in the core axial direction and new fuel can be loaded in the end at the opposite side to the end at the taken out side.
In CANDLE burning, there is no need for adjustment of criticality. Further, even if not adjusting the power density distribution, the power density distribution is maintained substantially constant. For this reason, there is the feature that from the beginning to the end of the operation cycle, there is no need for control of the reactivity of the core such as manipulation of the control rods. Further, there is the feature that the reactivity coefficient does not change and it is not necessary to change the method of operation together with burning.